Structural symmetry of viruses

viruses

1. Structural symmetry of viruses By- K omal P atil P riya K umari O mila Devi

2. TYPES OF VIRAL STRUCTURAL SYMMETRY: Helical Icosahedral Complex Envelope

3. HELICAL SYMMETRY Helical capsids are shaped much like hollow tubes with protein walls. The tobacco mosaic virus provides a well-studied example of helical capsid structure. A single type of protomer associates together in a helical or spiral arrangement to produce a long, rigid tube, 15 to 18 nm in diameter by 300 nm long.

4. The RNA genetic material is wound in a spiral and positioned toward the inside of the capsid where it lies within a groove formed by the protein subunits. Not all helical capsids are as rigid as the TMV capsid . Influenza virus RNAs are enclosed in thin, flexible helical capsids folded within an envelope .

5. The size of a hel ical capsid is influenced by both its protomers and the nucleic acid enclosed within the capsid . The diameter of the capsid is a function of the size, shape, and interactions of the protomers . The nucleic acid determines helical capsid length because the capsid does not seem to extend much beyond the end of the DNA or RNA.

6. ICOSAHEDRAL SYMMETRY Viruses employ the icosahedral shape because it is the most efficient way to enclose a space . A few genes , sometimes only one, can code for proteins that self assemble to form the capsid . In this way a small number of linear genes can specify a large three-dimensional structure.

7. Hexagons pack together in planes and cannot enclose a space, and therefore pentagons must also be used . When icosahedral viruses are negatively stained and viewed in the transmission electron microscope, a complex icosahedral capsid structure is revealed. The capsids are constructed from ring- or knob-shaped units called capsomers , each usually made of five or six protomers.

8. STRUCTURE OF ICOSAHEDRAL SYMMETRY

9. Pentamers (pentons) have five subunits; hexamers (hexons) possess six. Pentamers are at the vertices of the icosahedron, whereas hexamers form its edges and triangular faces. Protomers join to form capsomers through noncovalent bonding. The bonds between proteins within pentamers and hexamers are stronger than those between separate capsomers.

10. COMPLEX SYMMETRY Although most viruses have either icosahedral or helical capsids, many viruses do not fit into either category. The poxviruses and large bacteriophages are two important examples . The poxviruses are the largest of the animal viruses ( about 400 - 200 nm in size)

11. They possess an exceptionally complex internal structure with an ovoid- to brickshaped exterior. The double-stranded DNA is associated with proteins and contained in the nucleoid, a central structure shaped like a biconcave disk and surrounded by a membrane. The T2, T4, and T6 phages that infect E. coli have been intensely studied.

12. Their head resembles an icosahedron elongated by one or two rows of hexamers in the middle and contains the DNA genome . The tail is composed of a collar joining it to the head, a central hollow tube, a sheath surrounding the tube, and a complex baseplate . The sheath is made of 144 copies of the gp18 protein arranged in 24 rings, each containing six copies.

13. In T-even phages , the baseplate is hexagonal and has a pin and a jointed tail fiber at each corner . The tail fibers are responsible for virus attachment to the proper site on the bacterial surface. Complex bacterial viruses with both heads and tails are said to have binal symmetry because they possess a combination of icosahedral (the head) and helical (the tail) symmetry.

14. Envelope symmetry of viruses: Many animal viruses, some plant viruses, and at least one bacterial virus are bounded by an outer membranous layer called an envelope. envelope proteins are coded for by virus genes and may even project from the envelope surface as spikes or peplomers.

15. Influenza virus:

16. These spikes may be involved in virus attachment to the host cell surface . Influenza virus is a well-studied example of an enveloped virus. Spikes project about 10 nm from the surface at 7 to 8 nm intervals.

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